The objective of Cosmic Invaders is to prevent the invaders from reaching the bottom of the screen. This is done by deploying defensive structures, accessible from the bottom right. Once deployed, towers will automatically fire on your enemies. Each costs money, which along with your remaining life is displayed in the bottom left. Towers can be sold, giving you back half of their original cost plus potentially valuable real estate, by selecting them with the left mouse button and clicking the demolish button. Right click cancels selection and placement. Pressing the escape key exits the game.

Laser turrets fire straight up every few seconds, dealing damage to the first invader the shot hits. Missile batteries fire seeking missiles at a slower rate, but they deal heavier damage, plus splash damage around them. The missiles explode if they fail to hit anything within a few seconds, although they still deal splash damage if they're close enough to anything when they do. Spark towers will deal heavy damage to a single invader, but have much shorter range.

There are planned features not yet in the game, such as the ability to upgrade towers once placed, more types of towers, more enemy types, and boss waves. Click on the paper clip icon to initiate download.